Corbyn initially refused to condemn the list but on Thursday he and McDonnell said they would apologise for the document, though they stressed it was a “factual” summary of criticisms by MPs.

Unleashing his strongest attack yet on his opponents, Smith likened the left-wing movement Momentum to the 1980s splinter group Militant, and said it wanted to drag Labour away from Parliamentary democracy “by fair means or foul”.

He also raised the prospect of banning Momentum, saying he didn’t want ‘a party within a party’, though adding a ban would be a matter for the NEC not the leader.

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Jeremy Corbyn

Smith said although Corbyn and McDonnell’s “rhetoric has softened” and campaigning was now digital, “the real intention is just as it has always been”.

“To take control of the Labour Party, bit by bit, seat by seat, by fair means or foul, and to drag us away from the centre left of politics, where our values and our voters reside, to the hard left they represent, and which the country rejects,” he said.

“Jeremy Corbyn and his allies in Momentum want to lead our party down a route away from Labour’s mainstream, Parliamentary tradition, and away from the voters.

“And it is certainly not honest for Momentum to preach a message of unity, while planning to deselect the 170 MPs who dare to disagree with their assessment of Labour’s chances of winning.

“Momentum are trying to get rid of good Labour MPs through deselection, while Jeremy’s leadership is threatening to get rid of good Labour MPs through electoral defeat at the hands of the Tories.”

McDonnell’s aides hit back, saying Smith’s “bitter” speech showed he was ‘so desperate’ that he had a ‘scorched earth policy’ to talk down the party.

The Pontypridd MP said that whatever the outcome of the leadership contest, which ends on September 24, Labour’s future was still at stake.

“Just as we had to fight Militant in the 1980s, we have to fight to save the Party we love today. We know because the phony war and phony rhetoric has been swept aside in these last few weeks, as their true intentions have been revealed.

“Momentum in Brighton and Liverpool - some of them exactly the same people as were in Militant all those years ago - organising to deselect a Labour MP. Threats to MPs all over the country.

john “A ‘deselection list’ circulated by Jeremy’s campaign, attacking Labour MPs. Disowned by the campaign. Apologised for by the campaign. And then defended, on stage at this week’s hustings, by Jeremy himself.”

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John McDonnell and Jeremy Corbyn

Smith said that it was clear that “Jeremy and his side-kick John McDonnell” had decided to remain “the hard left protesters they’d been throughout their careers”.

Smith then referenced Paul Mason as proof of a Corbyn supporter keen on removing Labour MPs who criticised him.

“One of Jeremy’s economic advisers – rumoured to be his chief adviser if he wins– openly calling for deselection of any MP who disagrees with the Leader,” Smith said.

“It’s part of a culture of bullying and intimidation that says to Labour MPs and Labour members that they have to shut up and get in line or get out. Exactly the same tactics used by Militant in the 1980s.

“There is nothing comradely about setting up party within a party. Still less in trying to use our movement as a host body, seeking to occupy it, hollow it out, until it’s outlived its usefulness, when you throw it aside like a dead husk.

“But as John McDonnell said of himself, some see Labour membership as “a tactic… If it’s no longer useful, move on”.

Later, on LBC, Smith suggested the Corbyn was a puppet of McDonnell. “I don’t know whether Jeremy is controlled or whether he’s just very influenced by his close supporters and allies,” he said.

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Corbyn speaking at a Momentum rally in London

A senior Momentum source told HuffPost UK: “Selections are a matter for local party members and affiliates, who have the right to choose their officers and representatives through agreed democratic procedures.

“It is ironic that some of those who tried to deselect Jeremy as leader, without a vote and without following agreed procedures, have decided that Labour members and affiliates having the right to select their candidates in accordance with party rules is somehow abusive.”

A Momentum spokesman said: “Momentum brings together the enthusiasm and excitement in the Labour Party caused by Jeremy’s leadership. We seek to strengthen the party and help it win elections by making it a more participatory, democratic and campaigning party.

“We will work with people and organisations right across the labour movement to achieve these ends and transform society in the interests of the many.”